This invention relates to a cutter blade mounted to an output shaft of an engine of a rotary lawn mower, for cutting grass by rotation of the output shaft and discharging grass clippings to the rear of the lawn mower.
A cutter blade for lawn mowers is known from, for example, Japanese Utility Model Registration No. 2519253 entitled xe2x80x9cRotary Cutter Blade for Lawn Mowerxe2x80x9d.
The known cutter blade has a middle mounting section to be fixed to an output shaft of an engine and a blade section extending from each longitudinal end of the mounting section. The blade section has a cutting edge for cutting grass on its rotationally leading edge. The blade section further includes an angled section with a rotationally trailing edge extending in a rear upward direction with inclination, for producing airflow for discharging grass clippings. The cutting edge splits airflow into two flows along the front surface and the back surface of the angled section. As the rotational speed of the cutter blade increases, the airflow along the back surface of the angled section tends to soon divert from the surface. In order to reduce rotational noise by preventing such airflow diversion, the angled section is formed with a plurality of elongate vent holes.
It is a common art for rotary lawn mowers to provide an angled section to a rotary cutter blade for discharging grass clippings. The provision of the angled section generates a negative pressure (vacuum region) behind the angled section. The negative pressure causes the airflow to divert, generating rotational noise. In order to reduce the rotational noise, the angled section is formed with a plurality of elongate vent holes as described above to thereby reduce the negative pressure generated behind the angled section.
However, further reduction of noise caused by lawn mowers is necessary for environmental improvements.
The present invention provides a cutter blade for lawn mowers, capable of generating sufficient airflow for discharging grass clippings and reducing rotational noise.
According to an aspect of the present invention, there is provided a cutter blade for a lawn mower with a motor, which comprises: a mounting section mountable to an output shaft of the motor and extending radially outward in opposite directions from the mounted portion; a cutting edge formed on a rotationally leading edge on each side of the mounting section; and an airlift curved from the cutting edge to a rotationally trailing edge and having a plurality of vent holes; wherein, the trailing edge is shaped in a waveform comprising a plurality of protrusions and a plurality of depressions formed between the protrusions, the one of the plurality of protrusions closest to an outer end of the cutter blade is greatest in width; and the plurality of vent holes are elongate holes arranged in lines extending from the vicinity of the protrusions of the trailing edge toward the leading edge.
Rotation of the cutter blade of the present invention generates a negative pressure (vacuum region) rotationally behind the airlift. Since the negative pressure causes rotational noise, the vent holes are provided in the airlift to reduce the negative pressure. The waveform of the trailing edge prompts the reduction of the negative pressure, reducing vortices, thereby reducing noise generated by the rotation of the cutter blade. For example, of the constituents of the waveform, the protrusions increase the function of swirling up grass clippings while the depressions weaken the swirling-up function. As compared with a case where the protrusions have an equal width, providing the greatest width to the protrusion positioned closest to the outer end of the cutter blade where the centrifugal force is greatest, improves the function of swirling up grass clippings.
Further, the vent holes arranged in lines formed in the airlift reduce the negative pressure in the vicinity of the rear surface of the protrusions of the airlift where the negative pressure is most intense. This directly reduces the negative pressure where it is most intense, limiting the rotational noise of the cutter blade.
When the vent holes in lines are arranged in an arc in parallel with a rotation circle of the cutter blade, the vent holes always face the rotation direction during the rotation of the cutter blade without interrupting airflow, limiting the rotational noise of the cutter blade. Each line of the vent holes comprises a plurality of vent holes in an embodiment.